This invention pertains generally to radio frequency (R.F.) mixers and particularly to wideband R.F. mixers.
It is common practice in the radar art to downconvert received radar signals to first intermediate frequency (I.F.) signals by passing such radar signals through an R.F. mixer. One known way of improving mixer conversion loss is to use a so-called image enhancement mixer. Unfortunately, this type of mixer suffers from relatively large conversion losses and is practical only when the first I.F. signal is at a relatively high frequency and the signal bandwidth is relatively narrow. In an enhancement mixer the circuitry of an R.F. mixer may be arranged so that reactive terminations are provided with the final result being that the image signal inherent in any mixing process contributes to the power of the I.F. signals out of such mixer.
In any type of image-enhancement mixers being discussed it is necessary that some sort of filtering be provided so that only the image frequency signals (and not the local oscillator signals) may pass to the reactive terminations. In order to attain the requisite degree of filtering the image and L.O. signals must differ appreciably in frequency, with the resultant effect that the I.F. frequency must be relatively high and the bandwidth of the R.F. mixer be relatively narrow.
Phased-type image enhancement mixers such as those described in articles entitled "A New Phased-Type Image Enhanced Mixer" by L. E. Dickens and D. W. Maki, Microwave Symposium Digest, MTT-S 1975, pages 149-151, and "Phasing Type Image Recovery Mixers" by T. M. Oxley, Microwave Symposium Digest, MTT-S 1980, pages 270-273, obviate the filtering problem presented by conventional image-enhancement mixers. However, such image-enhancement mixers comprise two separate balanced mixers so circuit complexity and the requisite local oscillator (L.O.) power are increased.